At the present state of the art, either photomultipliers or photodiodes are used almost exclusively in different variations for converting weak high-frequency optical signals. Although conventional photodiodes convert the light current practically free of noise into an electrical current, the currents for low light quantities are however so small that the noise of a downstream amplifier is greater by a multiple. Photomultipliers and special avalanche photodiodes have an internal amplification so that even very weak light signals can be detected. Both components have however the disadvantage that they, in turn, add a noise component to the light signal, that they become very expensive and become impractical because of the requirement of a high-voltage source during use and that at least the photomultiplier is too large in a mechanical sense for many applications.
If the high-frequency optical signal is a periodic signal as is mostly the case for distance-measuring devices, then the noise ratio can be improved subsequently by means of an averaging procedure. This averaging takes place in the present state of the art primarily with respect to the analog electronic signal (for example by means of a narrow band filter) but this averaging is also possible in the digital area if the analog electronic received signal is first converted analog to digital.
The last-mentioned situation is disclosed in published German patent application 3,620,226 wherein an avalanche photodiode is used. All of these methods have, however, in common that the signal improvement by means of averaging is only then performed when the weak optical signal has already been charged with additional noise by converters and electronic amplifiers. This additional noise can be attenuated by averaging but cannot be fully eliminated with a reasonable effort. Furthermore, the necessary electronic components (optoelectronic converters, amplifiers and, if necessary, fast analog/digital converters) are expensive components and often not free of problems.